The present invention relates to injection molding machines, and in particular to such a machine which is capable of processing both thermoplastic compounds and glass loaded thermosetting polyester compounds, such as bulk molding compounds, with easy conversion between the two modes of operation.
Plastics materials which are becoming increasingly important and more widely used are thermosetting polyester compounds reinforced with glass fibers, and other types of fibers. An important application of components made of this type of molding compound is in the automotive industry where improved mileage requirements have necessitated the substitution of plastic parts for those which have customarily been made of metal. For example, the headlamp housing and front trim component for automobiles has customarily been made of a chrome plated metal, but, due to the strength and rigidity afforded by fiber reinforced bulk molding compounds (BMC), this component can be made entirely of such a material. Although this type of component can be made by compression molding or transfer molding techniques, these techniques are not particularly suited to high production due to the high degree of hand labor required and the long cycle time. The advantages of injection molding glass reinforced polyester compounds are faster cycle time, the elimination of hand labor, and a higher quality product having a better finish.
One of the disadvantages to making products of a fiber loaded polyester molding compound is reduction of strength of the product due to fiber breakage. One such thermosetting material is bulk molding compound, which comprises a thermosetting plastics material and glass fibers premixed to form a starting material having a dough-like consistency. Although the use of conventional screw machines are advantageous from the standpoint of short cycle times, by feeding the bulk molding compound into the barrel through the normal feed inlet which is at the rear end of the barrel, the shear forces developed by a conventional plasticizing screw or even a deeper flighted conveying screw tend to break the glass fibers. This results in lower product strength because the short, broken fibers are not as effective a reinforcing constituent as are longer fibers.
In order to minimize fiber breakage, it is desirable to limit, as much as possible, the distance in the barrel through which the material is conveyed. One technique for converting a standard injection molding machine from thermoplastic operation to glass loaded thermosetting operation is to replace the longer thermoplastics barrel with a shorter barrel. One problem with this is that it may take several days to make this conversion, thereby resulting in substantial machine down time and making conversion unfeasible except for very long runs. Furthermore, this requires the customer to stock both a thermoplastics barrel and screw and a barrel and screw specifically adapted for glass loaded thermosetting compounds, an expense which is not affordable unless a substantial portion of the processor's output is such thermosetting compounds.